Towards System-Oriented Formal Verification of Local-First Access Control

2026-04-26

AI Summary

This work addresses the lack of formally verifiable, fine-grained access control mechanisms in local-first systems operating at scale under low-trust collaboration settings. We propose a bottom-up approach that integrates a capability-based authorization model with Hashed Chronicle—a replicated data type—to design a Byzantine fault-tolerant collaborative group management mechanism. For the first time, system-level formal verification is introduced into local-first access control by leveraging the Verus framework to specify and verify a Rust implementation with zero runtime overhead. We formalize the semantics and key invariants of a simplified CRDT and prove the correctness of the core authorization logic, thereby providing Matrix, Keyhive, and similar systems with an integrable, high-assurance security foundation.

Technology Category

• Data Mining & Knowledge Management: Representing, Reasoning, and Using Provenance, Trust
• Multiagent Systems: Mechanism Design
• Knowledge Representation and Reasoning: Argumentation

Application Category

• Security and Privacy: Authentication, authorization, and access control
• Systems and Infrastructure for Web, Mobile and WoT: Decentralized Web and Fediverse systems
• Responsible Web: Data and user privacy-enhancing technologies for the Web

Abstract

Conflict-free replicated data types (CRDTs) and the local-first concept are increasingly employed not only in small-scale collaboration systems among few users who trust each other, but also in large-scale systems, like Matrix for instant messaging and Keyhive for collaborative documents. Since mutual trust is no longer warranted, these systems require Byzantine fault tolerance and fine-grained access control. As of today, Matrix and Keyhive pair an informal specification with an unverified reference implementation. In this work, we follow a bottom-up approach towards constructing formally verified authorization algorithms for Byzantine fault-tolerant local-first systems. As intermediate target for formal verification, we contribute semantics and invariants of a replicated data type for managing simplified collaboration groups, based on capabilities for access control and hash chronicles for replication. To enable future integration into local-first systems like Matrix and Keyhive, we strive for accessibility to system engineers by using the Rust programming language for formal specification, verification, and implementation, enabled by the Verus framework using the Z3 theorem prover at zero runtime cost. We report on our experience and preliminary results following this approach, and discuss next steps towards scaling up access control expressiveness. Whether this approach can be scaled up to the complexity of real-world local-first access control systems like Matrix or Keyhive remains future work, but our findings demonstrate the potential of system-oriented formal verification with Verus.

Problem

• local-first
• access control
• Byzantine fault tolerance
• formal verification
• CRDTs

Innovation

• local-first
• formal verification
• CRDT
• Byzantine fault tolerance
• Verus